How about if you change the Child
class to this:
class Child(db.Model):
id = db.Column(db.Integer, primary_key = True)
name = db.Column(db.String(120))
parent_id = db.Column(db.Integer, db.ForeignKey('parent.id'))
parent = db.relationship('Parent', backref=db.backref('children', lazy='dynamic'))
I don't know much about this, and I don't know if you need the back reference, but this setup works for me with Flask-Admin.